Irreversible Structural Phase Transition in [(9-triptycylammonium) ([18]crown-6)][Ni(dmit)2]: Origin and Effects on Electrical and Magnetic Properties

Materials exhibiting irreversible phase transitions, leading to changes in their properties, have a potential for novel application in electronic components such as a non-rewritable high-security memory. Here, we focused on the two salts, [(9-triptycylammonium)([18]crown-6)][Ni(dmit)₂] ( 1 ) and [(9-triptycylammonium)([15]crown-5)][Ni(dmit)₂] ( 2 ), which featured 2D sheet structures with alternately stacked cation and anion layers. Both salts exhibit similar cation arrangements, however, their anion arrangements differ significantly. The temperature-dependent magnetic susceptibilities of 1 and 2 were well reproduced by the alternating chain model (J_AC1/k_B=−306(8), J_AC2/k_B=−239(3) K) and the Curie-Weiss model (θ=−3.9(1) K), respectively. 1 experience a reversible phase transition around 40–60 K, causing anomalies in magnetic behavior. Moreover, an irreversible single-crystal-to-single-crystal phase transition to 1′ undergo at ~381 K, inducing a rearrangement of [Ni(dmit)₂]⁻ anions and a resistivity decrease from 6.5×10⁶ to 6.5×10² Ω cm. The susceptibility curve of 1′ was reproduced by a combination of the Curie-Weiss and dimer models (J_dimer/k_B=−407(5), θ=−26.7(5) K). The irreversible transition of 1 is the first example for such supramolecule and [Ni(dmit)₂]⁻ system to our knowledge, in opening potential new-type materials.     

(Nitronyl Nitroxide)‐Substituted Trioxytriphenylamine Radical Cation Tetrachlorogallate Salt: A 2p‐Electron‐Based Weak Ferromagnet Composed of a Triplet Diradical Cation

The synthesis and the solid state magnetic properties of (nitronyl nitroxide)‐substituted trioxytriphenylamine radical cation tetrachlorogallate, NNTOT+·GaCl4? , are reported. In the temperature region between 300 and 3 K, the magnetic behavior is characterized by the strong intramolecular ferromagnetic interaction (J/k_B=+400 K) between the radical ( NN ) and the radical cation ( TOT ⁺) and the weak intermolecular antiferromagnetic interaction (J/k_B=?1.9 K) between NNTOT⁺ ions. Below 3 K, a 3D‐type long‐range magnetic ordering into a weak ferromagnet was observed (T_N=2.65 K). The magnetic entropy (S_mag=8.97 J K^?1 mol^?1) obtained by the heat capacity measurement is in good agreement with the theoretical value of R ln3=9.13 J K^?1 mol^?1 based on the S=1 state.     

[CoII2L(NCS)2(SCN)2]: The First Cobalt Complex to Exhibit Both Exchange Coupling and Spin Crossover Effects

A spin crossover transition (350 K, μ_eff=3.15 μ_B per Co atom; 4.5 K, μ_eff=0.70 μ_B per Co atom) and weak antiferromagnetic exchange (2 J=−11.7 cm⁻¹) between the octahedral, doubly pyridazine bridged cobalt(II ) ions are features of the structurally characterized complex [Co^II₂L(NCS)₂(SCN)₂] (the ligand L is shown).     

Magnetism in organic radical ion salts based on nitronyl nitroxide derivatives substituted with heterocyclic aromatic hydrocarbons

Radical cation and anion salts of the neutral organic radicals, 2-imidazolyl nitronyl nitroxide (2-IMNN) and 2-benzimidazolyl nitronyl nitroxide (2-BIMNN), have been prepared and their magnetic properties studied by SQUID magnetometry. The radical salts exhibit one-dimensional (1-d) antiferromagnetic (AFM) intermolecular interactions with the exchange coupling J/k between −0.8 and −6.3 K, which are significantly reduced from those observed in the two neutral radicals, while 2-IMNN shows an AFM interaction with J/k=−88 K within the molecular dimers and 2-BIMNN has quasi 1-d ferromagnetic (FM) intermolecular interactions with J/k=+22 K (intrachain) and zJ′/k=+0.24 K (interchain). The magnetic properties of the nitronyl nitroxide and iminonitroxide derivatives having molecular structure related to 2-IMNN have also been investigated. In 2-benzimidazolyl iminonitroxide (2-BIMIN), the FM interaction observed in 2-BIMNN is replaced by strong 1-d AFM interaction with J/k=−11.7 K.     

Rational Design of a Confacial Pentaoctahedron: Anisotropic Exchange in a Linear ZnIIFeIIIFeIIIFeIIIZnII Complex

The first confacial pentaoctahedron comprised of transition metal ions namely Zn^IIFe^III_AFe^III_BFe^III_AZn^II has been synthesized by using a dinucleating nonadentate ligand. The face-sharing bridging mode enforces short Zn^II⋅⋅⋅Fe^III_A and Fe^III_A⋅⋅⋅Fe^III_B distances of 2.83 and 2.72 Å, respectively. Ab-initio CASSCF/NEVPT2 calculations provide significant negative zero-field splittings for Fe^III_A and Fe^III_B with |D_A|>|D_B| with the main component along the C₃ axis. Hence, a spin-Hamiltonian comprised of anisotropic exchange, zero-field, and Zeeman term was employed. This allowed by following the boundary conditions from the theoretical results the simulation in a theory-guided parameter determination with J^xy=+0.37, J^z=−0.32, D_A=−1.21, E_A=−0.24, D_B=−0.35, and E_B=−0.01 cm⁻¹ supported by simulations of high-field magnetic Mössbauer spectra recorded at 2 K. The weak but ferromagnetic Fe^III_AFe^III_B interaction arises from the small bridging angle of 84.8° being at the switch from anti- to ferromagnetic for the face-sharing bridging mode.     

Pressure effects of a genuine organic crystalline ferromagnet dupeyredioxyl

We have revealed that the isothermal magnetization M of the genuine organic crystalline dupeyredioxyl (N,N′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane; T_c(0)=1.48 K) observed below 10 K converges on the S=1 Brillouin function B₁((H+λM)/k_BT) with λ=2.4±0.2 or 2zJ/k_B=3.6±0.3 K, where J and z are, respectively, the averaged exchange interactions and coordination numbers for the S=1 spin system. This fact suggests that S=1 is constructed within a molecule via a strong ferromagnetic coupling between two S=1/2 spins on each of the two NO moieties. The modified notation of the Rushbrooke and Wood theory, T_c=2AzJS(S+1)/k_B (A=0.23±0.02 for the three-dimensional Heisenberg systems), is found to quantitatively hold not only for this S=1 spin system but also for other S=1/2 ferromagnets β-phase p-NPNN (2zJ/k_B=3.6 K) and 2,5-DFPNN (2zJ/k_B=2.8 K). Pressure effects of this compound have been studied under the hydrostatic pressure (P) up to 15 kbar. T_c(P) is revealed to show a negative pressure effect with the initial gradient a=d(T_c(P))/dP=−0.047 kbar⁻¹, nearly the same value for other organic ferromagnets as β-phase p-NPNN (−0.048 kbar⁻¹) and p-Cl-TEMPO radical (−0.03 kbar⁻¹), in contrast to the positive pressure effect for genuine antiferromagnets such as TANOL (a=+0.15 kbar⁻¹). Microscopically, different from the above two ferromagnets, the pressure-induced destruction of the orthogonality of molecular orbitals associated with the two NO moieties plays an effective role in reducing the intramolecular ferromagnetic interaction J₀. The possible weaker intermolecular interactions other than J₀ and J are also expected to be more susceptible to the stress of pressure to result in the reduction of their values perhaps even changing their sign, just as in the case of β-phase NPNN or p-Cl-TEMPO.     

Solvothermal Synthesis of [Cr7S8(en)8Cl2]Cl3 ⋅ 2H2O with Magnetically Frustrated [Cr7S8]5+ Double-Cubes**

A novel transition metal chalcohalide [Cr₇S₈(en)₈Cl₂]Cl₃ ⋅ 2H₂O, with [Cr₇S₈]⁵⁺ dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi- and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant, θ=−224(2) K, obtained from Curie-Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions between S=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)⁷=16384 microstates from seven Cr³⁺ centers, a simplified model with only two exchange constants was used for simulations. Density-functional theory (DFT) calculations yielded the two exchange constants to be J₁=−21.4 cm⁻¹ and J₂=−30.2 cm⁻¹, confirming competing AFM coupling between the shared Cr³⁺ center and the peripheral Cr³⁺ ions of the dicubane cluster. The best simulation of the experimental data was obtained with J₁=−20.0 cm⁻¹ and J₂=−21.0 cm⁻¹, in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr³⁺ ions as compared to the AFM exchange between the central and peripheral Cr³⁺ ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal-chalcogenide clusters into polymeric networks.     

(Azulene‐1,3‐diyl)‐bis(nitronyl nitroxide) and (Azulene‐1,3‐diyl)‐bis(iminonitroxide) and Their Copper Complexes

In contrast to diradicals connected by alternant hydrocarbons, only a few studies on those connected by nonalternant hydrocarbons have been reported. The syntheses, structures, and magnetic properties of azulene‐1,3‐diyl linked bis(nitronyl nitroxide) (NN₂Az) and bis(iminonitroxide) (IN₂Az) diradicals and their Cu(hfac)₂ (hfac=hexafluoroacetylacetonate) complexes were investigated. NN₂Az was shown to have an intramolecular ferromagnetic interaction with J _obs/k _B=+10.0 K (H =−2J S ₁ ⋅S ₂) between (nitronyl nitroxide) spins, whereas IN₂Az was estimated to have a much weaker intramolecular magnetic interaction. The reactions of NN₂Az and IN₂Az with Cu(hfac)₂ gave a 1:2 [{Cu(hfac)₂}₂(NN₂Az)] complex and a 1:1 [Cu(hfac)₂(IN₂Az)] ⋅ C₆H₁₂ complex, respectively. [{Cu(hfac)₂}₂(NN₂Az)] showed strong intramolecular antiferromagnetic interactions (J _1‐Cu‐R/k _B≈−800 K, J _2‐Cu‐R/k _B≈−500 K) between the Cu^II spins and the coordinating NN spins, whereas [Cu(hfac)₂(IN₂Az)] exhibited a ferromagnetic exchange interaction (J _obs‐Cu‐R/k _B=+114 K) between the Cu^II spin and the imino‐coordinated iminonitroxide spin.     

Redox-Induced Modulation of Exchange Interaction in a High-Spin Ground-State Diradical/Triradical System

A triplet ground-state diradical molecule, bis(nitronyl nitroxide)-substituted diphenyldihydrophenazine ( 1 ^..), that can be converted into a one-electron oxidized species, 1 ^{… +} , in the quartet ground state has been developed. Surprisingly, these species, 1 ^.. and 1 ^{… +} , can be used under ambient conditions because they are reasonably stable under aerobic conditions, even in solution. The temperature-dependent magnetic susceptibilities reveal that 1 ^.. and 1 ^{… +} are in the triplet state, with a weak exchange interaction (J₁/k_B = +3.1 K) and quartet ground state with a strong exchange interaction (J₂/k_B = +160 K), respectively. The interconversion between the neutral and one-electron oxidized species can be realized through electrochemical reactions. Significantly different absorption bands in the near-IR region newly appeared in the electronic spectra acquired during electrochemical oxidation/reduction.     

Reaching Optimal Light‐Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

Ground‐state (GS) and excited‐state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES (³An*); 2) imino‐nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10. Improving the efficiency of the PMMDs involves strengthening the ES intramolecular exchange coupling (J_ES) between transient and persistent SCs, hence the choice of 2‐pyrimidinyl (pm) as B elements to replace the original p‐phenylene (ph). Dissymmetry of the pm connectors leads to [SC‐B‐P‐B‐SC] regio‐isomers int. and ext., depending on whether the pyrimidinic nitrogen atoms point towards the An core or the peripheral SCs, respectively. For the int. regio‐isomers we show that the photoinduced spin alignment is significantly improved because the J_ES/k_B value is increased by a factor of more than two compared with the ph‐based analogue (J_ES/k_B>+400 K). Most importantly, we show that the optimal J_ES/k_B value (≈+600 K) could be reached in the event of an unexpected saddle‐shaped structural distortion of the lowest ES. Accounting for this intriguing behavior requires dissection of the combined effects of 1) borderline intramolecular steric hindrance about key An–pm linkages, which translates into the flatness of the potential energy surface; 2) spin density disruption due to the presence of radicals; and 3) possibly intervening photochemistry, with An acting as a light‐triggered electron donor while pm, IN, and OV behave as electron acceptors. Finally, potentialities attached to the [(SC)‐pm‐An‐pm]_int pattern are disclosed.     

Weak Antiferromagnetic Exchange and Ferromagnetic Alignment of FeII (S=2) Spins in Differently Charged {HAT ⋅ (FeIICl2)3}n (n=2- and 3-) Assemblies of Hexaazatriphenylenes (HAT)

Hexaazatrianthracene (HATA) and hexaazatriphenylenehexacarbonitrile {HAT(CN)₆} are reduced by metallic iron in the presence of crystal violet (CV⁺)(Cl⁻). Anionic ligands are produced, which simultaneously coordinate three Fe^IICl₂ to form (CV⁺)₂{HATA ⋅ (Fe^IICl₂)₃}²⁻ ⋅ 3 C₆H₄Cl₂ ( 1 ) and (CV⁺)₃{HAT(CN)_6. (Fe^IICl₂)₃}³⁻ ⋅ 0.5CVCl ⋅ 2.5 C₆H₄Cl₂ ( 2 ). High-spin (S=2) Fe^II atoms in both structures are arranged in equilateral triangles at a distance of 7 Å. An antiferromagnetic exchange is observed between Fe^II in {HATA ⋅ (Fe^IICl₂)₃}²⁻ ( 1 ) with a Weiss temperature (Θ) of −80 K, the PHI estimated exchange interaction (J) is −4.7 cm⁻¹. The {HAT(CN)₆ ⋅ (Fe^IICl₂)₃}³⁻ assembly is obtained in 2 . The formation of HAT(CN)₆^.3− is supported by the appearance of an intense EPR signal with g=2.0037. The magnetic behavior of 2 is described by a strong antiferromagnetic coupling between the Fe^II and HAT(CN)₆^.3− spins with J₁=−164 cm⁻¹ (−2 J formalism) and by a weaker antiferromagnetic coupling between the Fe^II spins with J₂=−15.4 cm⁻¹. The stronger coupling results in the spins of the three Fe^IICl₂ units to be aligned parallel to each other in the assembly. As a result, an increase of the χ_MT values is observed with the decrease of temperature from 9.82 at 300 K up to 15.06 emu ⋅ K/mol at 6 K, and the Weiss temperature is also positive being at +23 K. Thus, a change in the charge and spin state of the HAT-type ligand to ⋅3⁻ results in ferromagnetic alignment of the Fe^II spins, yielding a high-spin (S=11/2) system. DFT calculations showed that, due to the high symmetry and nearly degenerated LUMO of both HATA and HAT(CN)₆, their complexes with Fe^IICl₂ have a variety of closely lying excited high-spin states with multiplicity up to S=15/2.     

Tetrabutylammonium Salts of Aluminum(III) and Gallium(III) Phthalocyanine Radical Anions Bonded with Fluoren‐9‐olato− Anions and Indium(III) Phthalocyanine Bromide Radical Anions

Reduction of aluminum(III), gallium(III), and indium(III) phthalocyanine chlorides by sodium fluorenone ketyl in the presence of tetrabutylammonium cations yielded crystalline salts of the type (Bu₄N⁺)₂[M^III(HFl−O⁻)(Pc^.3−)]^.−(Br⁻) ⋅ 1.5 C₆H₄Cl₂ [M=Al ( 1 ), Ga ( 2 ); HFl−O⁻=fluoren‐9‐olato⁻ anion; Pc=phthalocyanine] and (Bu₄N⁺) [In^IIIBr(Pc^.3−)]^.− ⋅ 0.875 C₆H₄Cl₂ ⋅ 0.125 C₆H₁₄ ( 3 ). The salts were found to contain Pc^.3− radical anions with negatively charged phthalocyanine macrocycles, as evidenced by the presence of intense bands of Pc^.3− in the near‐IR region and a noticeable blueshift in both the Q and Soret bands of phthalocyanine. The metal(III) atoms coordinate HFl−O⁻ anions in 1 and 2 with short Al−O and Ga−O bond lengths of 1.749(2) and 1.836(6) Å, respectively. The C−O bonds [1.402(3) and 1.391(11) Å in 1 and 2 , respectively] in the HFl−O⁻ anions are longer than the same bond in the fluorenone ketyl (1.27–1.31 Å). Salts 1 – 3 show effective magnetic moments of 1.72, 1.66, and 1.79 μ_B at 300 K, respectively, owing to the presence of unpaired S= 1/2 spins on Pc^.3−. These spins are coupled antiferromagnetically with Weiss temperatures of −22, −14, and −30 K for 1 – 3 , respectively. Coupling can occur in the corrugated two‐dimensional phthalocyanine layers of 1 and 2 with an exchange interaction of J /k _B=−0.9 and −1.1 K, respectively, and in the π‐stacking {[In^IIIBr(Pc^.3−)]^.−}₂ dimers of 3 with an exchange interaction of J /k _B=−10.8 K. The salts show intense electron paramagnetic resonance (EPR) signals attributed to Pc^.3−. It was found that increasing the size of the central metal atom strongly broadened these EPR signals.     

Cyclic Triradicals Composed of Iminonitroxide–Gold(I) with Intramolecular Ferromagnetic Interactions

A triangular gold(iminonitroxide‐2‐ide) trimer complex ( 5 ) was prepared and investigated to determine its magnetic properties. The results showed that the metalloid triradical is highly stable, even in solution under aerated conditions. The intramolecular exchange interaction of 5 was found to be positive (J_intra/k_B≈+29 K), thus showing that 5 is in a quartet ground state. In addition, a silver sandwich complex ( 5 ‐Ag⁺‐ 5 ) was prepared and its electronic and magnetic properties were also clarified.     

Eu(O2C-C≡C-CO2): An EuII Containing Anhydrous Coordination Polymer with High Stability and Negative Thermal Expansion

Anhydrous Eu^II–acetylenedicarboxylate (EuADC; ADC²⁻ = ⁻O₂C-C≡C-CO₂⁻) was synthesized by reaction of EuBr₂ with K₂ADC or H₂ADC in degassed water under oxygen-free conditions. EuADC crystallizes in the SrADC type structure (I4₁/amd, Z=4) forming a 3D coordination polymer with a diamond-like arrangement of Eu²⁺ nodes (msw topology including the connecting ADC²⁻ linkers). Deep orange coloured EuADC is stable in air and starts decomposing upon heating in an argon atmosphere only at 440 °C. Measurements of the magnetic susceptibilities (μ_eff=7.76 μ_B) and ¹⁵¹Eu Mössbauer spectra (δ=−13.25 mm s⁻¹ at 78 K) confirm the existence of Eu²⁺ cations. Diffuse reflectance spectra indicate a direct optical band gap of E_g=2.64 eV (470 nm), which is in accordance with the orange colour of the material. Surprisingly, EuADC does not show any photoluminescence under irradiation with UV light of different wavelengths. Similar to SrADC, EuADC exhibits a negative thermal volume expansion below room temperature with a volume expansion coefficient α_V=−9.4(12)×10⁻⁶ K⁻¹.     

Ferromagnetic Interactions in a 1D Alternating Linear Chain of π‐Stacked 1,3‐Diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl Radicals

X‐ray studies show that 1,3‐diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 6 ) adopts a distorted, slipped π‐stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7‐(thien‐2‐yl)benzotriazin‐4‐yl 6 show two fully reversible waves that correspond to the ?1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie–Weiss behavior in the 5–300 K region with C=0.378 emu K mol^?1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12 cm^?1, J₂=+1.28 cm^?1).     

Kinetic study of the ligand substitution on the trimethylplatinum(iv) complexes with unidentate ligands

Ligand substitution kinetics for the reaction [Pt^IVMe₃(X)(NN)]+NaY=[Pt^IVMe₃(Y)(NN)]+NaX, where NN=bipy or phen, X=MeO⁻, CH₃COO⁻, or HCOO⁻, and Y=SCN⁻ or N₃⁻, has been studied in methanol at various temperatures. The kinetic parameters for the reaction are as follows. The reaction of [PtMe₃(OMe)(phen)] with NaSCN: k₁=36.1±10.0 s⁻¹; ΔH₁^≠=65.9±14.2 kJ mol⁻¹; ΔS₁^≠=6±47 J mol⁻¹ K⁻¹; k₋₂=0.0355±0.0034 s⁻¹; ΔH₋₂^≠=63.8±1.1 kJ mol⁻¹; ΔS₋₂^≠=−58.8±3.6 J mol⁻¹ K⁻¹; and k₋₁/k₂=148±19. The reaction of [PtMe₃(OAc)(bipy)] with NaN₃: k₁=26.2±0.1 s⁻¹; ΔH₁^≠=60.5±6.6 kJ mol⁻¹; ΔS₁^≠=−14±22 J mol⁻¹K⁻¹; k₋₂=0.134±0.081 s⁻¹; ΔH₋₂^≠=74.1±24.3 kJ mol⁻¹; ΔS₋₂^≠=−10±82 J mol⁻¹K⁻¹; and k₋₁/k₂=0.479±0.012. The reaction of [PtMe₃(OAc)(bipy)] with NaSCN: k₁=26.4±0.3 s⁻¹; ΔH₁^≠=59.6±6.7 kJ mol⁻¹; ΔS₁^≠=−17±23 J mol⁻¹K⁻¹; k₋₂=0.174±0.200 s⁻¹; ΔH₋₂^≠=62.7±10.3 kJ mol⁻¹; ΔS₋₂^≠=−48±35 J mol⁻¹K⁻¹; and k₋₁/k₂=1.01±0.08. The reaction of [PtMe₃(OOCH)(bipy)] with NaN₃: k₁=36.8±0.3 s⁻¹; ΔH₁^≠=66.4±4.7 kJ mol⁻¹; ΔS₁^≠=7±16 J mol⁻¹K⁻¹; k₋₂=0.164±0.076 s⁻¹; ΔH₋₂^≠=47.0±18.1 kJ mol⁻¹; ΔS₋₂^≠=−101±61 J mol⁻¹ K⁻¹; and k₋₁/k₂=5.90±0.18. The reaction of [PtMe₃(OOCH)(bipy)] with NaSCN: k₁ =33.5±0.2 s⁻¹; ΔH₁^≠=58.0±0.4 kJ mol⁻¹; ΔS₁^≠=−20.5±1.6 J mol⁻¹ K⁻¹; k₋₂=0.222±0.083 s⁻¹; ΔH₋₂^≠=54.9±6.3 kJ mol⁻¹; ΔS₋₂^≠=−73.0±21.3 J mol⁻¹ K⁻¹; and k₋₁/k₂=12.0±0.3. Conditional pseudo-first-order rate constant k₀ increased linearly with the concentration of NaY, while it decreased drastically with the concentration of NaX. Some plausible mechanisms were examined, and the following mechanism was proposed. [Note to reader: Please see article pdf to view this scheme.] © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 523–532, 1998     

Synthesis and Magnetic Properties of (Pyrrolidin‐1‐oxyl)–(Nitronyl Nitroxide)/(Iminonitroxide)‐Dyads

Unlike extensively studied diradicals linked by π‐conjugated systems, only a few studies have investigated weakly coupled diradicals linked by an sp³ carbon atom. Herein, we prepared pyrrolidin‐1‐oxyl–(nitronyl nitroxide)‐dyad 5 and pyrrolidin‐1‐oxyl–iminonitroxide‐dyad 6 . From the observed temperature dependence of the magnetic susceptibility, 5 and 6 were determined to be in singlet ground states with 2J_intra/k_B=?35.2 K and ?13.6 K, respectively. From these results and theoretical calculations of related diradicals, the spin‐polarization model counting the small spin density of the sp³ carbon atom could be used as a spin‐prediction model.     

Field-Induced SMM Behaviour of Tetranuclear Coordination Clusters with a Cubane [Ni4O4] Core

Two tetranuclear Ni(II) complexes: [Ni₄(HL¹)₄] ⋅ H₂O ( 1 ) and [Ni₄(HL²)₄] ⋅ 1.5 dmf ( 2 ) where dmf=dimethylformamide, H₃L¹=4-(tert-butyl)-2-(((2-hydroxy-5-nitrophenyl)imino)methyl)-6-(hydroxymethyl)phenol and H₃L²=4-(tert-butyl)-2-(hydroxymethyl)-6-(((2-hydroxyphenyl)-imino)methyl)phenol, have been prepared and characterized by single crystal X-Ray diffraction, elemental analysis and FT-IR spectroscopy. The solid-state structures reveal the formation of highly symmetric and asymmetric [Ni₄O₄] cubane cores in complexes 1 and 2 , respectively. Extensive magnetic studies show that both complexes present ferromagnetic exchange interactions between the Ni(II) ions within the cubane core with g=2.113(3), J₁=−7.89(8) cm⁻¹, J₂=13.3(1) cm⁻¹ and |D|=11.3(4) cm⁻¹ (for 1 ) and g=2.206(4), J₁=1.0(1) cm⁻¹, J₂=7.8(1) cm⁻¹ and |D|=8.7(2) cm⁻¹ (for 2 ). The large anisotropy, high ground spin state (arising from the ferromagnetic coupling) and the good isolation of the clusters provided by the Schiff base ligands, give rise to the first examples of field-induced single-molecule magnets (FI−SMM) in Ni₄O₄ clusters and to the highest energy barrier reported to date in a Ni₄O₄ cluster.     

Syntheses and Properties of (Nitronyl nitroxide)-substituted Tri-phenylamine ortho-Bridged by Two Oxygen and Sulfur Atoms

A nitronyl nitroxide unit ( NN ) was linked with a triphenylamine-based condensed polycyclic skeleton DOTT to form a radical substituted donor NN - DOTT . X-ray crystal structure analysis demonstrated a flat bowl shape of the DOTT unit. EPR spectra showed the localization of electron spin on the NN unit. Chemical oxidation of the DOTT unit produced radical-substituted radical cation salts NN - DOTT ⁺ ⋅ SbF₆⁻ and NN - DOTT ⁺ ⋅ FeBr₄⁻ that are stable under ambient conditions. The magnetic behavior of NN - DOTT ⁺ ⋅ SbF₆⁻ is characterized by the strong intramolecular ferromagnetic interaction between NN and DOTT ⁺. The X-ray structural analysis of NN - DOTT ⁺ ⋅ FeBr₄⁻ shows planar structure of DOTT and 1D mixed-stack column of NN-DOTT ⁺ and FeBr₄⁻. Magnetic measurements established that NN - DOTT ⁺ ⋅ FeBr₄⁻ undergoes magnetic phase transition into a weak ferromagnet at 7 K.